The shortcomings of conventional puffer interrupters or gas circuit breakers (GCBs) has been the subject of continuous investigation. (e.g., IEEE paper 81 SM 413-4, An Improvement of Low Current Interrupting Capability in Self Interruption GCB, by Murari et al, submitted on Feb. 2, 1981). European Patent Application No. 0019806 (co-invented by two of the authors of the aforementioned IEEE paper) describes several embodiments of a circuit interrupter having an improved interrupting capability in the low current region. This interrupter uses a negative pressure zone formed by detaching operation of the contacts of the interrupter as a sort of "suction assist".
So-called negative pressure interrupters cause arc-interrupting gas flow by using: a piston, attached to a moving contact and located in a generally cylindrical chamber, to create a negative pressure zone, relative to the region across which the arc is formed when the interrupter is opened; and a nozzle of flow channel joining the two pressure zones. Thus, the separation of two interrupter contacts causes a pressure differential to be formed across the nozzle. This pressure differential will, in turn, cause a mass flow of interrupting gas, such as sulfur hexafloride (SF.sub.6) which cools and extinguishes the arc at current-zero.
Conventional devices utilize a construction wherein the arc is formed within a closed volume, the arcing or upstream chamber relative to the flow nozzle. The flow nozzle is the only source of escape for the high pressure gas formed therein. If the temperature of the gas remains constant, as under low current conditions, then the pressure in the arcing chamber will decrease as gas is removed. Since the pressure within this chamber or zone decreases as the gas flows out of the flow nozzle, the mass and consequently the pressure of the gas in the adjacent negative pressure region increases. Therefore, it is necessary that the rate of change of volume in the negative pressure region increases with time in order to maintain a pressure differential. Mathematically, this relationship is expressed by: EQU V=LA
where V is the volume, L is the length of the negative pressure region or downstream chamber, and A is the cross sectional area of the chamber. For all practical purposes, in the case of cylindrical geometry, the cross sectional area, A, is a constant and the length of downstream chamber is a function of time. Thus, the rate of change of volume is: EQU dV/dt=AdL/dt=f(t)
In order for the rate of change of volume to increase with time, dL/dt must increase with time. The only way to gain an increasing dL/dt is to cause the piston to move at a greater rate of speed. This usually requires that the energy of the prime mover increase with time. In addition to being very costly, such an approach detracts from the advantages of gas circuit breakers utilizing the negative pressure principle to improve their operation in the low-current interrupting region. Since the prime mover of an interrupter represents a significant portion of the total cost of the device, a reduction of the required energy output from the prime mover will proportionately reduce the overall cost of the apparatus.
Another shortcoming of conventional self-flow generating interrupters utilizing a suction assist for improving low-current interruption operation, is the size of the arcing zone. The size of the arcing zone determines, for all practical purposes, the overall size of the interrupter. In other words, the size of the interrupter is determined, in large part, by the maximum current interruption capability of the device. Thus, the arcing zone is larger than what is needed for medium and smaller current interruption. More importantly, the performance of the interrupter in the mid and low interrupting current region is less than optimal.
It should be clear, therefore, that self-flow generating gas interrupters which utilize the negative pressure principle have heretofore been a compromise between what would be the optimal design of the interrupter over the entire range of currents that one would expect to flow through the device. Clearly, improvement is in order.